MIMO (Multiple-Input Multiple-Output) and transmit diversity techniques have provided attractive solutions for increasing downlink capacity in wireless mobile networks and currently MIMO is being considered for WCDMA (Wideband Code Division Multiple Access) standardization for downlink transmission.
For various reasons, uplink multi-antenna transmission solutions have not been given much attention to date. In order to keep the costs and complexity of such handsets acceptable, it has been proposed that they only include a single full transmit (ie, uplink) chain and two receiver (ie, downlink) chains.
Lack of signal diversity can be a problem when transmitting common channels in a mobile telecommunications system. In the case of a mobile handset attempting to use the random access channel (RACH) procedure to establish an uplink (or two-way) connection, soft handover is not possible and thus no macro diversity gain can be obtained. The diversity problem exists if there is no (or only a small amount of) multipath or time diversity available in the radio channel.
Typical steps involved in the RACH procedure are shown in FIG. 1. An initial preamble P0 is sent by user equipment UE at a first, relatively low power. The downlink power is measured and the transmit power of the initial preamble P0 is set (based on the DL measurement) with the proper margin due to the open loop inaccuracy (open loop power control not being particularly accurate, since it is difficult to measure large power dynamics accurately in terminal equipment). Transmission of the first and subsequent preambles is commenced at the beginning of any of a number of pre-defined time slots, known as access slots. There are 15 access slots per two frames, and they are spaced 5120 chips apart. After transmitting the first preamble P0, the UE decodes the acquisition Indication Channel (AICH) to determine whether the base station BS has successfully received the preamble. In the event the first preamble P0 is not acquired by the BS, the power level is raised (typically by 1 dB) and a second preamble P1 sent at the new power level in the next available access slot. The AICH is decoded to check whether the BS received P1, and the process is repeated for further preamble signals, until a power level is reached where the AICH indicates reception of the last transmitted preamble at the BS.
Once a preamble is acknowledged by the BS, the UE transmits the 10 ms or 20 ms long message part that consists of control and data parts of the RACH transmission (transmission power of the message part is typically based on that of the latest (successful) preamble). For example, an IP address can be carried using the data part.
Whilst the UE is performing the RACH procedure, it is possible that the radio link between the UE and the BS is in deep fade. This means that multiple sequential preambles (with increased transmit power) must be transmitted to compensate the channel attenuation and to get the acceptable performance for the RACH reception. This increases the time taken for the RACH process to be successful, and in the meantime generates more interference to the other users of the RAN (Radio Access Network). Power consumption is also increased at the UE, which has a deleterious effect on battery life.